The invention relates to a fuel injection system for internal combustion engines. It includes an air flow meter device in which a pivotal flap is disposed in the induction tube and moves in reaction to the air flow through the induction tube. A restoring force is exerted against the flap in the opposite direction. The degree of rotation of the flap is a measure of the air flow rate. The invention particularly relates to apparatus of the type in which, for low air flow rates, the flap operates essentially in dependence of the pressure differential across the flap, while, during the transition to larger air flow rates, at least a portion of the flap also acts as an air foil where that portion of the flap experiences air flow on both sides and thus is subject to a lifting force and a corresponding torque around its pivotal axis.
In known injection systems which operate with an air measuring flap, the measurement is subject to an error which is commonly referred to as an averaging error and is caused by the fact that the average of the air pressure fluctuations due to the engine operation causes the air flap to be displaced further than would be the case without such pulsations, i.e., corresponding to the actually aspirated air quantity. Furthermore, in these known systems it is relatively difficult and expensive to make allowance for particular parameters which should affect the fuel-air ratio (the value of .lambda.). In all known systems, the preparation of the fuel-air mixture also constitutes a serious difficulty.